1. Field of the Invention
The invention relates to a control device for a vehicle drive train equipped with a torque converter.
2. Description of the Related Art
A known control device for a vehicle drive train equipped with a torque converter is, for example, described in Japanese Patent Application Publication No. 7-139613 (JP-A-7-139613). The control device for a vehicle drive train, described in JP-A-7-139613, calculates the speed ratio of the torque converter on the basis of a torque converter characteristic, that is, a capacity coefficient, by referring to a predetermined relationship. The thus calculated speed ratio is, for example, used to calculate the input shaft rotational speed of an automatic transmission.
Incidentally, in the existing control device for a vehicle drive train, when the torque converter has such a characteristic that the capacity coefficient becomes a local maximum at a predetermined speed ratio in the relationship between the speed ratio and the capacity coefficient, there is a problem that the speed ratio cannot be calculated within part of a speed ratio variation range. Specifically, the relationship between the speed ratio and capacity coefficient of the torque converter generally has such a characteristic that the capacity coefficient increases from a predetermined value to the local maximum as the speed ratio increases from zero and, then, the capacity coefficient decreases to a value smaller than the predetermined value as the speed ratio further increases. Therefore, if such a characteristic is used to calculate the speed ratio, the speed ratio is not uniquely determined for capacity coefficients, other than the local maximum, larger than or equal to the predetermined value in a low speed ratio variation range in which the speed ratio is smaller than or equal to a speed ratio corresponding to the predetermined value, and, therefore, the speed ratio cannot be calculated. In contrast to this, when the speed ratio is not calculated on the basis of the capacity coefficient, it is impossible to calculate the output shaft rotational speed of the torque converter using the speed ratio. Therefore, it is necessary to provide a sensor for detecting the output shaft rotational speed. This leads to an increase in cost for manufacturing a vehicle.